1. Field of the Invention
The invention relates to a discharge lamp of the short arc type which is used for a projection light source and for a projector. The invention relates particularly to a xenon discharge lamp of the short arc type of the direct current operation type.
2. Description of the Related Art
A so-called discharge lamp of the short arc type having an anode and a cathode opposite one another is used as a light source lamp in a projection device for a demonstration and in a projector device. In this discharge lamp the so-called flicker phenomenon arises in which the deflection of the arc increases in the course of operation of the lamp. When the flicker phenomenon occurs, the images projected onto the screen flicker; which is perceived as unpleasant in visual observation. When the flickering occurs, the short arc lamp is replaced and this time that flickering is confirmed is referred to as the flicker service life.
It is known that the above described flicker phenomenon is caused by electrode wear and turbulence of the gas flow in the arc tube. Conventionally, for lamps used for the above described purposes, various techniques have been proposed for suppressing the flicker phenomenon.
The following techniques are known:    a technique in which the tip area of the cathode is carbonized, thus the motion of the emitter substance to the tip area of the cathode is accelerated and thus the wear of the tip area of the cathode is reduced, as described in Japanese Patent No. 2782611.    a technique in which the cathode material, with tungsten as the main component, is changed so that the amount of change of shape is reduced and thus the stability of the arc is maintained, as described in Japanese Patent No. 2851727.
Another technique in which electrodes for a flicker-free lamp are produced is the technique described in Japanese Patent Application No. 2002-93363.
Additionally, a technique is also known in which in order to stabilize the gas flow in the arc tube in the upper area of the arc tube an outside cooling device is employed for providing cooling air which cools, convection is suppressed and the arc is stably maintained. The use of the outside cooling device, however, often causes enlargement of the light source device which is considered undesirable. Furthermore, the gas pressure within the arc tube is reduced by excessive cooling.
Another technique is known in which by improving the electrode shape the influence of convection is reduced. For example, in U.S. Pat. No. 6,614,186 a short arc lamp is described in which for the anode in the connecting area between the forward region of the tip surface and the body there is a peripheral projection with a V-shaped cross section.
In a projector device with a high light intensity, such as a DMD (digital mirror device), having pixels of the reflection type of liquid crystals and the like, a xenon lamp of the short arc type with a kW range, high radiance and high light intensity filled with xenon gas as the discharge medium is advantageously used. This xenon lamp also suffers from the lack of durability of the lamp due to the formation of flicker.
Recently there has been a demand for especially high radiance in a small DMD with high precision. The xenon lamps are becoming common in such uses and those lamps have the distance between the electrodes which is becoming smaller and smaller and, further, the gas filling pressure has been increased, e.g., to ≧4×106 Pa (computed at 25° C.). When the distance between the electrodes becomes smaller, a temperature increase of the cathode results which leads to premature wear. In particular, in a xenon lamp turbulence of the gas flow arises principally in the arc tube. When a change in convection occurs, the arc is induced to fluctuate. In these xenon lamps, as a result of the increased of the gas pressure, the effect of convection becomes greater which results, due to the mutual action and synergistic effect of both the cathode wear and the convection turbulence, in the flicker phenomenon occurring prematurely.
In a short arc lamp used in the above lamps, it has been discovered by the inventors that an improvement of convection within the arc tube occurs, and, further, that a relationship between the convection and flicker phenomenon exists which is described below. It is noted that in this description, the lamp is limited only to a short arc lamp of the type used in the above described field, i.e., to a short arc lamp which is operated with a horizontal position of the tube axis of the lamp. Therefore, this description is not pertinent for a lamp operated with a vertical position.
FIGS. 12(a) and 12(b) show, in an enlarged view, the state of convection of a xenon lamp in the prior art. Specifically, in FIG. 12(a) the lines between the anode 81 and the cathode 82 constitute the arc shape, and the arrows represent the state of gas convection within the arc tube 83. Since the speed of the added gas is accelerated by the pressure difference between the front side of the cathode spot and the vicinity of the anode remote from the cathode 82 in a direction toward the anode 81, the gas advances between the electrodes essentially parallel to the arc tube axis. The gas which has been accelerated by the arc flows along the essentially cylindrical anode 81 to behind this anode 81. At the same time, the gas tries to move to above the arc tube since the gas is heated by the arc.
In the initial stage, the gas flow—in the direction of the arc tube axis along the portion of the body having a uniform diameter which constitutes the maximum outside diameter of the anode 81—moves away from the anode 81 (hereinafter also called simply “deportion”), returns again to the middle area of the arc tube 83 which makes the gas flow turbulent. Influenced by the turbulence of this flow, a fluctuation occurs in the arc, although the amount of it need not be problematical. This fluctuation of the arc accelerates the wear and drying-out of the emitter of the cathode 82.
FIG. 12(b) illustrates that over the course of operation of the lamp the tip of the cathode 82 is heavily worn and the emitter substance is also dried out. The result of which is that the fluctuation of the arc gradually increases towards the end of the lamp service life. As a result, at the start of operation, the gas flow which had deportioned from the body of the anode 81 now becomes turbulent due to the greater fluctuation of the arc, and the gas flow begins to deportion in the corner area 81 a on the border between the tapering region of the tip area of the anode 81 and the region of the anode with the maximum outside diameter. The turbulence of gas flow convection in the vicinity of the arc is therefore greatly influenced by the fluctuation of the arc. Consequently, the arc together with the cathode enters an extremely unstable state towards the end of the service life.
As was described above, due to the influence of the wear of the cathode, the drying-out of the emitter substance and the turbulence of gas flow convection, the flicker phenomenon arises prematurely which in turn leads to a shortening of the service life of the lamp. In the prior art, a plurality of measures had been taken to eliminate electrode damage. Currently, however, the situation is such that even using the electrode above it is difficult to prolong the flicker service life.
Still further when a cooling device is used for improving convection in the above described manner, the operating property of the lamp can still changes, and solution is also difficult to implement in practice.
In the above described U.S. Pat. No. 6,614,186, upon placing a projection in the electrode tip area of the arc tube an eddy is formed such that in the vicinity of the arc the speed of the gas flow is reduced, and thus the effect of convection is thereby reduced. However, the flow energy is weakened by the generation of the eddy due to the projection. Since the flow departs from the projection area and since turbulence begins to form in the flow after departure, arc fluctuations arise, towards the end of the service life of the lamp, due to the turbulence of convection when cathode wear occurs. Ultimately, the flicker service life is not prolonged.